The present invention relates to a gear shift arrangement for a vehicle with a main change-speed gear, a clutch which is disengaged when a gear change is effected, and a load shift gear which is operable while the vehicle is under load.
In U.S. patent application, Ser. No. 302,327, filed 15 Sept. 1981, now U.S. Pat. No. 4,438,657, 1981 there is described therein an arrangement for controlling the main change-speed gear and the load shift gear. The load shift gear can be shifted under load between a normal position and an underspeed position in which there is a speed reduction of, say, 20%, and a corresponding increase in tractive force of, say, 25 to 30%. The load shift gear enables the driver of a tractor to deal with a momentary resistance, causing the engine to labor by shifting temporarily into underspeed. Unnecessary main gear changes are avoided. A single gear lever works in a gate for selecting the main gear. Moreover, the gate is pivotally mounted and linked to the load shift gear. In any main gear, the gear lever can be swung laterally to shift between normal and underspeed positions of the load change gear.
Since the load change gear is designed to operate under load, it operates, if correctly used, under synchronized conditions. It is not desirable to have the load change gear operate outside these conditions because it can lead to an increase in the relative speed of rotation of parts and thereby increase the demands made upon the synchronizing elements of the main gear. In the aforesaid application, it is already proposed to exclude the possibility of operation of the load change gear when the main gear is in neutral. In the first place, lateral movement of the gear lever in neutral takes place freely in the cross-slot of the gate without pivotal movement of the gate. In the second place, inadvertent movement of the gate in neutral, (e.g. because foreign matter provides the gear lever moving freely in the gate), is blocked by a mechanical interlock between the gear lever and the selector linkage for the load change gear.
This prior design does not, however, prevent simultaneous actuation of the main gear shift and the load change gear and, if the driver inadvertently, in going through the gate, also actuates the load change gear, (double actuation), severe synchronization problems are liable to occur.